1. Field of the Invention
The present invention generally relates to apparatuses for and methods of reducing noise in high-frequency band, and more particularly, to an apparatus and a method which are employed in an optical video disc player for correcting frequency distribution caused by a pickup and transmission distortion.
2.Description of the Background Art
One of the existing optical video disk players for optically reproducing an NTSC color video signal optically recorded on a video disk record is disclosed at large in "Laser Disc Technical Book" issued 1st Nov. 1986 by Kabushiki Kaisha ASCII.
As shown in frequency spectrums of FIGS. 1 and 2A, on that optical video disc record, there is recorded a modulated video signal with an FM deviation area, an upper sideband and a lower sideband in the range of 4 MHz to 13.5 MHz which has been obtained by FM-modulating the NTSC color video signal to have a sink tip of 7.6 MHz, a pedestal level of 8.3 MHz and a white peak of 9.3 MHz. 0n this optic video disc record, there are also recorded a 2.3 MHz carrier and a 2.8 MHz carrier that never fail to have been FM-modulated by audio signals on two channels. Additionally, on a low-frequency side of the FM audio signal, there is recorded in a frequency multiplexing manner a digital audio signal with a format identical to that of a compact disc record.
While recently, there exists an increasing demand for a fine-pitch image, in order to meet that demand with the present rotating speed and thus central frequency of a disc unchanged, expansion of the video signal in frequency band is required. For this purpose of expanding the video signal in band, three methods can be proposed as shown in FIGS. 2B to 2D.
FIG. 2B shows a method (first method) of expanding both the low- and high-frequency bands by eliminating the FM audio signal. FIG. 2C shows another method (second method) of expanding only the high-frequency band and doubling energy thereof while maintaining the FM audio signal in the present state. FIG. 2D shows still another method (third method) of expanding only the low-frequency band and doubling energy thereof by eliminating the FM audio signal. According to the second and third methods, only the high-frequency band or the low-frequency band is to be doubled in energy, so that video signal processing at RF stage in recording on a disc becomes more complicated than that in the first method. Therefore, implementation of a filter having such frequency characteristics as shown in FIGS. 2C and 2D is impossible in terms of technology. Consequently, it is most desirable in general to implement a fine-pitch image by employing the first method.
Meanwhile, when optical reproduction of the signal is made with the use of a conventional pickup, a spot of laser beam which is larger than a pit width on the disc reduces resolution of signal detection, resulting in a deteriorated high-frequency band component of the reproduced RF signal as shown in FIG. 3. In a CAV (Constant Angular Velocity) disc operative at a constant angular velocity, such a tendency becomes more evident with a position closer to the inner circumference of the disc and a higher frequency. In the CAV disc, linear velocity varies from 10.75 m/s to 32 m/s between the inner circumference and the outer circumference. Therefore, even with signals of the same pit length, length of a signal detected in the outer circumference per unit time is larger than that in the inner circumference. Length of a signal detected in the outer-most circumference per unit time is three times as large as that in the inner-most circumference. Accordingly, the high-frequency band component of a reproduced RF signal is degraded more considerably in a position closer to the inner circumference of the CAV disc.
Meanwhile, in a CLV (Constant Linear Velocity) disc operative at a constant liner velocity, the same degradation of the high-frequency band as seen in the inner circumference of the CAV disc appears in the full circumference.
As described above, since the RF signal obtained through a pickup entails considerable degradation of frequency characteristics in the high-frequency band, a method disclosed in pp. 77 to 79 of the above-mentioned "Laser Disc Technical Book" has been hitherto employed. This method is shown in FIG. 4. Referring to FIG. 4, an RF signal having been detected by a pickup 31 is amplified by a preamplifier 32. In order to correct the degradation of frequency characteristics in the high-frequency band, a high-frequency band amplification correcting circuit 33 is interposed between the preamplifier 32 and a limiter 34. The high-frequency band amplification correcting circuit 33 corrects high-frequency band amplification by raising the high-frequency band component of the RF signal. An output signal of the limiter 34 is demodulated into a color video signal by an FM demodulating circuit 35.
Such correction of the high-frequency band amplification as described above has, however, the following problems. As mentioned above, frequency distribution of a recorded signal in a conventional optical video disc is shown in FIG. 2A. Since in recording, the FM-modulated signal is passed through the limiter, the reproduced RF signal contains a double wave and a triple wave as shown in FIG. 5. Furthermore, distortion in a transmission system and a circuit system also causes the double wave through the pickup. Such a double wave may possibly overlap the high-frequency band of a basic wave as shown in FIG. 5. Particularly, in case of a fine-pitch video disc record which has a video signal band expanded in both the low- and high-frequency bands in frequency distribution of a recorded signal as shown in FIG. 2B, expansion of signal waves is equal to that of conventional signal waves plus hatched portions in FIG. 5. As a result, overlap of the basic wave and the second wave becomes more apparent. If such a signal is detected, or FM-demodulated without any processing in reproduction, then such overlap appears on an image plane as beat or noise.
Furthermore, since resolution of signal detection is low in the inner circumference of the CAV disc or in the full circumference of the CLV disc, it is inherently hard for the high-frequency band component to appear in reproduction. Such correction as made by raising this high-frequency band component may deteriorate S/N, so that picture quality is rather deteriorated.
Subsequently, signals with degraded high-frequency band components as will be obtained from the inner circumference of the CAV disc or the full circumference of the CLV disc will be described in detail with reference to FIGS. 6A to 6D and 7A to 7D.
FIGS. 6A to 6D show frequency spectra of RF signals which are not corrected for the high-frequency band amplification. As shown in FIG. 6A, the RF signal is degraded in the high-frequency band including a carrier C. When such an RF signal is passed through the limiter, as shown in FIG. 6C, an upper sideband component H and a lower sideband component L are averaged before output. While ratio of the carrier to the sidebands remains substantially unchanged, a double wave W and a beat Y caused by overlap of the double wave and the basic wave, which have been seen only in the high-frequency band, are average. Therefore, when an output signal of the limiter is FM-demodulated, beat and noise are generated in the high-frequency band as shown in FIG. 6D.
FIGS. 7A to 7B show frequency spectra of RF signals which are corrected for the high-frequency band amplification. Likewise in FIG. 6A, the RF signal shown in FIG. 7A is corrected by the high-frequency band amplification correcting circuit 33 such that the high-frequency band component is raised. This also causes the beat Y which have been seen only in the high-frequency band to be amplified. Thereafter, the RF signal is passed through the limiter 34 so that both the noise and the beat are amplified as shown in FIG. 7C. FM-demodulation of such an RF signal will result in degradation of S/N and amplification of beat in the high-frequency band.
Meanwhile, in FIGS. 6A to 6D and 7A to 7D, O represents an audio beat, and FIG. 6B is entirely the same as FIG. 6A.
As described above, the degradation of the high-frequency band component can be prevented by simply raising the high-frequency component of a reproduced RF signal, but instead, S/N is degraded and beat is built up in the high-frequency band, disturbing the image plane.